The calcium-sensing receptor (CaSR) is a G-protein-coupled receptor (GPCR) that signals through the activation of phospholipase C, increasing levels of inositol 1,4,5-triphosphate and cytosolic calcium. The CaSR belongs to the subfamily C of the GPCR superfamily, which also includes receptors for glutamate, gamma aminobutyric acid (GABA), pheromones and odorants that all possess a very large extra-cellular domain. This domain is highly negatively charged and is involved in binding of calcium and other positively charged molecules. The CaSR is found in the parathyroid glands but has also been identified in the brain, intestine, pituitary, thyroid glands, bone tissue and kidneys [Brown, E., M. Calcium-Sensing Receptor. Primer of the Metabolic Bone Diseases and Disorders of Mineral Metabolism Fifth Edition, 2003 by American Society for Bone and Mineral Research, Chapter 17, p. 111.; Drueke, T. E. Nephrol Dial Transplant (2004) 19, suppl 5, v20-v26].
The calcium-sensing receptor (CaSR) detects changes in extra-cellular calcium concentration and initiates the functional response of this cell, which is a modulation of the secretion of the parathyroid hormone (PTH). Secretion of PTH increases extra-cellular calcium ion concentration by acting on various cells, such as bone and kidney cells, and the extra-cellular calcium ion concentration reciprocally inhibits the secretion of PTH by acting on parathyroid cells. The reciprocal relationship between calcium concentration and PTH level is an essential mechanism for calcium homeostasis maintenance.
The calcimimetic activity corresponds to the ability to produce or induce biological responses observed through variations in the concentration of extracellular calcium ions (Ca2+)e and extracellular magnesium ions (Mg2+)e.
(Ca2+)e and (Mg2+)e ions play a major role in the body since they regulate calcium homeostasis on which the vital functions of the body depend. Thus, hypo- and hypercalcemia, that is to say conditions in which (Ca2+)e ions are below or above the mean threshold, have a major effect on many functions, such as cardiac, renal or intestinal functions. They deeply affect the central nervous system (Chattopadhyay et al. Endocr. Review, 1996).
It has been shown that Ca2+ and Mg2+ ions, but also Ba2+ ions, within millimolar concentration ranges, stimulate CaSRs. Activation of CaSRs might be induced in the brain by β-amyloid peptides, which are involved in neurodegenerative diseases such as Alzheimer's disease [Ye et al, J. Neurosci. Res., 47, 547-554, 1997].
Disturbance of CaSR activity is associated with biological disorders such as primary and secondary hyperparathyroidism, osteoporosis, cardiovascular, gastrointestinal, endocrine and neurodegenerative diseases, or certain cancers in which (Ca2+)e ions are abnormally high.
Primary hyperparathyroidism (primary HPT) is characterised by elevated levels of PTH and serum calcium which is typically caused by adenoma or hyperplasia of the parathyroid gland. It can result in bone pain and excessive bone resorption.
Secondary hyperparathyroidism (secondary HPT) often develops in patients who have reduced kidney function and is characterised by elevated levels of PTH. The underlying causes are complex, but a reduced ability to convert vitamin D to calcitriol and elevated levels of phosphorus play significant roles in the development of secondary HPT. If left untreated, the clinical manifestations of secondary HPT include bone and joint pain and limb deformities [Harrington, P. E. and Fotsch, C. Calcium Sensing Receptor Activators: Calcimimetics. Current Medicinal Chemistry, 2007, 14, 3027-3034].
A reduced kidney function or renal failure is also accompanied by renal osteodystrophy, e.g. osteitis fibrosa, osteomalacia, adynamic bone disease, or osteoporosis. The disorders are characterized by either high or low bone turnover. Osteoporosis is a multifactor disease which depends in particular on age and sex. While menopausal women are very greatly affected, osteoporosis is increasingly proving to be a problem in elderly men, and, for the moment, no really satisfactory treatments exist. Its social cost may become even heavier in the years to come, particularly as life expectancy is becoming longer. Osteoporosis is currently treated with estrogens, calcitonin or biphosphonates which prevent bone resorption without stimulating bone growth. More recent data demonstrate that intermittent increases in PTH or in derivatives thereof are effective in the treatment of osteoporosis and make it possible to remodel bone by stimulating bone formation [Whitfield et al., Drugs & Aging 1999 August; 15 (2): 117-129 1999]. This new therapeutic approach for treatment of osteoporosis appears to be very advantageous, although major problems are associated with the use of PTH hormone, such as the route of injection, but also the appearance of tumours, observed recently during clinical trials in humans. Intermittent secretion of endogenous PTH can be obtained by blocking the calcium sensing receptor. The blocking of PTH secretion with CaSR agonists may be followed by a rapid increase in PTH (rebound effect), which is then beneficial in the treatment of osteoporosis.
A compound having an activating effect on CaSR (CaSR agonist), that is, a compound which selectively acts on CaSR to mimic or strengthen the action of Ca2+, is called a calcimimetic. On the other hand, a compound having an antagonistic effect on CaSR (CaSR antagonist, that is, a compound which suppresses or inhibits the action of Ca2+), is called a calcilytic.
The calcium-sensing receptor has recently been found to be a potent target for developing therapeutic options such as use of calcimimetics for treatment of diarrhea. [Osigweh et al, J American Coll. of Surgeons, V201, Issue 3, suppl 1, September 2005, p 17.]
Calcimimetics have been shown to be commercially useful for the treatment of hyperparathyroidism (HPT): The calcimimetic compound Cinacalcet® [Balfour, J. A. B. et al. Drugs (2005) 65(2), 271-281; Lindberg et. al. J. Am. Soc. Nephrol (2005), 16, 800-807, Clinical Therapeutics (2005), 27(11), 1725-1751] is commercially available for the treatment of secondary HPT in chronic kidney disease patients on dialysis and for the treatment of primary HPT in patients with parathyroid carcinoma. Thus, proof of concept for activators of calcium sensing receptor (CaSR) in humans has been achieved and the clinical relevance is well established.
Other calcimimetic compounds were for example described in WO02/059102, WO98/001417, WO05/065050, WO03/099814, WO03/099776, WO00/21910, WO01/34562, WO01/090069, WO97/41090, U.S. Pat. No. 6,001,884, WO96/12697, EP1203761, WO95/11221, WO93/04373, EP1281702, WO02/12181, WO04/56365, WO04/069793, WO04/094362, US2004242602, WO04/106280, WO04/106295, WO04/106296, WO05/068433, WO05/115975, EP 1757582, WO 2009/051718 and WO2010/021351.